Hi Mike. Thanks for the diagram. Following the heating wiring from the programmer to terminal 9 of the wiring centre, then terminal 3 and through the room stat then to terminal 4 and on to the 3 port valve. That's all fine and working but I can't see how that will fire the boiler unless that's supposed to make the orange live to terminal 6 then 12 to boiler switched live. Or does the grey also need to be live from the cylinder stat satisfied before the 3 port will make the orange live for heating?
It's not very clear from this diagram, but the re should be an "installer link" between 7 & 8. The grey should effectively a permanent live fed either from the HW off on the programmer when heating only is selected, or the satisfied terminal in the cylinder stat when both heating and hot water are selected.
I’ll go back tomorrow to check the grey. Assuming it is live as you describe, should the orange switch live to fire the boiler when the programmer calls for heating only?
As already explained, if you have live feed to the boiler from another source, it will backfeed up the orange wire, but go nowhere as the switch will be open.